Here's a pretty good discussion from the Pet Education
part of Foster's & Smith:

"Over-supplementation of calcium during pregnancy may increase the risk of eclampsia. There is a complex way the body maintains the proper amount of calcium in the blood. The body is constantly adding calcium to bones and then removing it, as needed. This is regulated by a hormone produced by the parathyroid gland, called parathyroid hormone. If a dog receives increased amounts of calcium during pregnancy, her body's production of parathyroid hormone greatly decreases. When the dog suddenly needs large amounts of calcium for milk production, the system is not ready to start removing it from the bone. This is because it takes some time for the parathyroid gland to start producing the hormone again. Because of the lack in parathyroid hormone, the blood calcium level suddenly drops, and produces the signs of eclampsia.

So, adequate amounts of calcium need to be given during pregnancy, but not enough to slow down the production of parathyroid hormone. This means calcium supplements are generally not recommended. Also, it is important for the calcium and phosphorus in the diet to be at the correct ratio of 1:1 (i.e.; 1 part calcium to 1 part phosphorus). Vitamin D must also be present in adequate amounts.

Once a dog has had milk fever, there is an excellent chance that she will also have it with future litters if preventive steps are not taken. Be sure to work closely with your veterinarian if your dog has had eclampsia in the past and is pregnant again.

In conclusion, it is of great importance for owners of pregnant or nursing dogs to be able to recognize the signs of eclampsia. If you feel your female dog is showing these signs, remove the pups to prevent further nursing and seek veterinary assistance at once."